Molded lamellar gypsum product

ABSTRACT

A gypsum molded structure which comprises gypsum having a lamellar structure in which the (010) planes are uniformly orientated parallel in one direction. The structure is obtained by causing a hydraulically setting material, comprising gypsum, to set as it is subjected, together with excess water, to pressure thereby to discharge the excess water.

United States Patent Taniguchi et al.

MOLDED LAMELLAR GYPSUM PRODUCT inventors: Takashi Taniguchi; Kunihiro Abe;

Takashi Sugimoto, all of Yokkaichi, Japan Mitsubishi Petrochemical Company Limited, Tokyo, Japan Filed: July 16, 1973 Appl. No.: 379,306

Assignee:

Foreign Application Priority Data July 19, 1972 Japan 47-71648 U.S. C1 161/162, 161/164, 161/270, 161/168,161/413,161/182,161/247,106/110 Int. Cl B32b 13/02 Field of Search 161/159, 164, 162, 168,

References Cited UNITED STATES PATENTS 10/1948 Parsons 106/88 1 Jan. 28, 1975 2,731,377 1/1956 Riddell 811211. 154/88 3,150,032 9/1964 RU1J8nSlC1n.... 161/161 3.459.571 8/1969 Shannon 106/114 3,481,829 12/1969 5111111 Jr. @1211 162/164 FOREIGN PATENTS OR APPLICATIONS 617,512 4/1961 Canada .161/164 Primary Examiner-W. J. Van Balen Assistant Examiner-Patricia C. Ives Attorney, Agent, or Firm-Robert E. Burns; Emmanuel J. Lobato; Bruce L. Adams [57] ABSTRACT A gypsum molded structure which comprises gypsum having a lamellar structure in which the (010) planes are uniformly orientated parallel in one direction. The structure is obtained by causing a hydraulically setting material, comprising gypsum, to set as it is subjected, together with excess water, to pressure thereby to discharge the excess water.

8 Claims, 11 Drawing Figures PATENTEBJANZ 81975 SHEET 1 BF 8 FIG.

FIG. 2

PATENIEDJAN28 I975 SHEET 3 BF 8 FIG. 4

O O O m m m m m m TIME (MIN) FIG. 5

i DAY) O O m m QE lkwzwmkm 0252mm IOO TIME (MIN) .PATENTEU 3,862,881

SHEET 5 OF 8 o 9 LL.

1) v I) v g 5 Q) y j a 7 42 c) 6 c) c) c) o 00 L0 r m PATENTED 3 882 881 sum ear 8 PATENTEDJAN28I975 3,862,881

SHEET 7 [1F 8 PATENTEUJAHNIQYS 3.862.881

SHEET 8 [IF 8 FIG. ll.

MOLDED LAMELLAR GYPSUM PRODUCT BACKGROUND OF THE INVENTION This invention relates to a method of producing a strong gypsum plaster mold having high density, particularly a gypsum plaster board from a-type or B-type calcined gypsum or anhydrous gypsum, and to a gypsum plaster board with a lamellar structure that cannot be found in the natural state.

According to the prior art, a gypsum plaster board is made by the steps of kneading a-form or B-form calcined gypsum or anhydrous gypsum with water, and in the case of anhydrous gypsum, together with a solidification promoter, and then solidifying it in a mold. However, the plaster board thus produced has disadvantageously a specific gravity of 1.1 at most and insufficient mechanical strength.

In the above case, the total quantity of the water added, for example, into calcined gypsum is merely based on the necessary quantity to solidify the gypsum plaster, namely 1.5 in mole ratio with respect to the plaster in accordance with the following reaction formula, and the minimum needed quantity to provide fluidity to the mixture of the plaster and water.

Further, it usually takes 40-60 minutes in the solidification 24 hours in the curing and drying, so that the process requires a rather long time to complete.

SUMMARY OF THE INVENTION The principal object of this invention is to provide a gypsum plaster board having improved mechanical strength and high density of gypsum, of lamellar structure, which is applicable for use as wall material, tiles, floor board, and the like, and a method of producing the same.

Another object of this invention is to provide a building board material which has a smooth surface and a good exterior appearance and is directly applicable as an external building material, and also is printable by various manners, for example anastati'c printing, and silk-screen printing, and to provide a method of producing the same.

A further object of this invention is to provide a method of producing building board material whereby relief designs can be imparted to the board.

It is still another object of this invention to provide a method of producing building board material whereby the solidification time can be shortened to one to two hundredth or less of that of the prior art.

Accordingly, in accordance with its broadest definition, this invention is characterized by a gypsum plaster molded material comprising gypsum having a lamellar structure wherein the (010) planes are uniformly oriented toward one direction in parallel.

Further, in accordance with another embodiment of this invention, there is provided a method of producing a gypsum plaster molded material having a lamellar structure. This method of producing a gypsum plaster moulded material comprising gypsum having a lamellar structure where the (010) planes are uniformly oriented toward one direction in parallel is characterized by the steps of mixing a hydraulically setting material water-solidifying substance calcined gypsum or anhydrous gypsum with water in a quantity greater than the theoretical quantity to set the hydraulically setting material and sufficient to keep the fluidity of the material and setting the mixture, applying the pressure at least while the hydraulically setting'material is fluid under the pressure in order that excessive water for the solidification be removed.

The nature, principle, and utility of the invention will be more clearly apparent from the following detailed description beginning with a consideration of the general aspects and features of the invention and concluding with specific examples of practice illustrating preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE ILLUSTRATIONS In the illustrations:

FIG. 1 is an electron-photomicrograph of a crystal of a-form calcined gypsum;

FIG. 2 is an electron-photomicnograph of crystals of B-form calcined gypsum;

FIG. 3 is an electron-photomicrograph of gypsum having a lameller structure;

FIG. 4 is a graphical representation of the relation between the compressive strength of a gypsum product according to this invention and the elapsed time;

FIG. 5 is a graphical representation of the relation between the bending strength of a. gypsum product according to this invention and the elapsed time;

FIG. 6 is a sectional view of one example of a mold;

FIG. 7 is a sectional view of another example of a mold;

FIG. 8 shows an X-ray diffraction chart of a molded gypsum product wherein X-ray incidence is perpendicular to the surface of the product to which pressure has been applied;

FIG. 9 shows an X-ray diffraction chart of a molded gypsum product wherein X-ray incidence is parallel with the surface of the product to which pressure has been applied;

FIG. 10 shows an X-ray diffraction chart of a molded product produced from a mixture of gypsum and portland cement (1:1) wherein X-ray incidence is perpendicular to the surface of the product to which pressure has been applied; and

FIG. 11 shows an X-ray diffraction chart of a molded product produced from a mixture of gypsum and portland cement (1:1) wherein X-ray incidence is parallel with the surface of the product to which pressure has been applied.

DETAILED DESCRIPTION This invention, in one aspect thereof, relates to a method of producing a gypsum plaster product, particularly a gypsum plaster board. This method is characterized by the process steps of mixing a hydraulically setting material comprising gypsum with excessive water, and removing the excess during the cause of setting of the hydraulically setting material under pressure.

By the practice of the method of this invention, the following advantages are afforded.

1. Operations are easier in kneading and molding because of the presence of the excessive water.

2. The solidification time, or the dehydration time, is shorter, even as short as 10 seconds, and curing and drying times are not necessary.

3. It is possible to produce a molded product some ten times stronger than the prior products within a short time, since according to this invention, the excessive water is removed under pressure during the solidification and the space after removal of water is filled with inorganic particles.

4. The original strength of the mold after solidification is enough for handling immediately after press dehydration.

5. The produced mold has improved dimensional stability with respect to heat and water since the distance between the inorganic particles has been shortened by pressure, and the attraction therebetween is larger.

6. The surface of products is smooth and is also printable since the mixture of gypsum and water is highly homogeneous due to the presence of excessive water and to ease of kneading and since the mixture is caused to setting under pressure.

7. The plaster board can have relief design (relievo or intaglio) thereon when a mold provided with corresponding relief pattern therein is used. According to the conventional silk-screen method, where relief design is formed by adhesion of powder of gypsum or the like by means of an adhesive, the operation is not easy, and the thickness of the relievo is 1mm at most because of the relievo construction being brittle. Although a sculpture method provides a thicker relievo, the process requires a harder operation. As shown above, according to this invention, the relief design can be imparted on the molded product in the process of molding, and the relievo construction is quite strong and has a thickness of about 10mm with a good appearance.

8. It is possible to produce a molded product with a desired specific gravity by changing the pressure and/or the quantity of inorganic filler, for example, gravel, fly ash, etc. In this invention it is preferable that the quantity of the water added into calcined gypsum and/or anhydrous gypsum be between the theoretical or stoichiometric quantity for the set of the gypsum and 50 times that quantity. When the quantity is more than 50 times the theoretically necessary quantity, the required mold is too large, and a large vessel to mix the water and gypsum therein is necessary; hence, it is not desirable from the economical standpoint. Considering the fluidity of the mixture when being poured into the mold, in general, the quantity of the water preferably stands between 1 and 10 times the theoretical quantity, and, more preferably between 4 and 10 times.

In this invention, the pressure be more than 1 kg/cm just in order to shorten the molding time, while is preferably more than 4 kg/cm to produce rather a strong product such as flooring material. The upper limit of is sucked from outside under reduced pressure in combination with the inner compression.

Further, the system under setting is preferably kept below 40C since the reaction of water and calcined gypsum and/or anhydrous gypsum is exothermic, and the strength of the product becomes worse when the temperature becomes high. For example, the tensile strength decreases by 20 percent at 40C as compared with that at 20C when setting the mixture of water'and ,B-form calcined gypsum after dehydration under pressure. The lower limit of the temperature is the freezing point of the system, particularly the temperature range between about 0C and 38C is preferred.

in order to set the hydraulically setting material in a mold under pressure while removing excess water, it is necessary to use a mold that is so constructed that only the water is removed therefrom leaving the hydraulically setting material therein. More particularly, at least one part of the mold, exemplarily the bottom thereof is made of porous material, such as, for examples, a sintered metal, or otherwise the bottom is provided with a water-excluding opening, such as, for example, a plurality of capillaries or perforation in communication between the inside and the outside of the mold. When the diameter of the opening is large, microporous material such as paper, paper board, fabrics, non-woven fabrics is possibly laid in the mold. The microporous material can be so used as to constitute the surface of the molded plaster product as an integral part thereof.

The pressure is applied at least during the period when the hydraulically setting material is fluid under the pressure.

It is to be understood that this invention is not limited by the following examples but may be variously embodied within the scope of the claims.

EXAMPLE 1 fl-form calcined gypsum and water were mixed and stirred in a weight ratio of 2:1 (2.5 times the theoretical quantity of water), and the mixture was poured into a press mold provided with a bottom made of a porous sintered metal, and was pressed within the mold to the bottom to remove excess water and set the gypsum whereby a gypsum plasterboard was produced.

Mechanical properties of the product thus produced were determined at the elapsed time of 0 and 24 hours after the solidification, are shown in Table 1.

Bending Strength in accordance with .115 K-6911-1970 (the same as in the following) Compression Strength in accordance with 11S K-691l-1970 (the same as in the following) the pressure is, from the economical point of view, about 500 kg/cm preferably about 200 kg/cm Additionally, some improvement is obtained when the water EXAMPLE 2 A mold provided with a large number of waterexcluding perforation in the bottom and with filter paper placed on the bottom was employed. The operations were the same as in Example 1. The results obtained are shown in Table 2.

the new gypsum by electron-photomicrograph. This is apparently different from those shown in FIG. I and FIG. 2.

Table 2 Run Pressure Specific Bending Strength Compression Strength Setting Time No. kg/cm Gravity 0 hr 24'hrs. 0 hr 24 hrs. second 3 4.0 1.7 57 152 I90 620 do.

4 2.0 67 I75 419 715 do.

5 100 2.1 72 I99 433 850 do.

EXAMPLE 3 In general, gypsum is classified into three classes in accordance with the content of water of crystallization. The compound may possibly be interchanged according to the following formula, and the molecule structure, crystal structure, and nature, etc. have been studied.

Table 3 Run Pressure Specific Bending Strength Compression Strength Setting Time No. kg/cm Gravity 0 hr 24 hrs. 0 hr 24 hrs. second 3 4.0 1.8 65 I69 369 695 do.

EXAMPLE 4 In this regard, CaSO .2H O, a-and B-CaSO,./2H G Table 4 and II and III-CaSO, are respectively classified under the name of crystalline or dihydrated gypsum, clacined gypsum, and anhydrous gypsum, and a-form of calcined gypsum differs from B-form in the crystal struc ture, and II and III of anhydrous gypsum differ from each other in the solubility in water; ll-type is not soluble, while III-type is soluble.

Bending Strength Compression Strength Run Pressure Specific kg/cm kg/cm Setting Time No. kg/cm Gravity 0 hr 24 hrs. 0 hr 24 hrs. second After studying the plaster board thus produced. we discovered that the crystal of gypsum thereof has a special structure which is different from the particle-type of the set gypsum (CaSO,.2H O) produced from a-form gypsum (CaSO H O) shown in FIG. 1 and the needle-type of the set gypsum (CaSO .2H O) produced from B-form gypsum (CaSOfl/ H O) shown in FIG. 2. The crystal is surprisingly in a lamellar structure (FIG. 3) that we believe has never been found before.

Accordingly. a gypsum having lamellar structural crystal is now originally produced, while a watersolidifying gypsum solid has been classified only into a-form or ,B-form. FIG. 3 shows the crystal structure of The known crystal structures are the particle-type (oz-form) or otherwise the needle-type (B-form), while the gypsum that has been set in accordance with this invention has lamellar structure, so it is considered that the molded gypsum product has a higher density, and better physical strength because of the stronger attraction between crystals, and can decrease the rate of water molecule entering into it. Actually various tests have illustrated the improvements of the hardness, compression strength, bending strength, etc.; gypsum that has been set a short time. It has also been found that the molded gypsum product in accordance with this invention does not lose its dimensional stability and CaSO . 2 heatlng 1n a1r or under reduced pressure/ of water Table 5 Type of Calcined Gypsum a [3 Real Specific Gravity 2.76 2.32 Water required 1%) 35 90 Compression Strength (kg/cm 450 500 I I40 Bending Strength (kg/cm) I50 40 Setting Time (min) l5 4 7 The quantity of water required for obtaining CaSO -2H O Physical properties of CaSO,'2H O, in accordance with the A.S.T M, C 26 Method,

As shown in Table 5, conventional process requires such longer setting time and the mechanical properties are not very good.

In addition to the above, data obtained by X-ray diffraction analysis clearly show the lamellar structure which comprises a plurality of laminas, and which structure is in accordance with the results from the electron-photomicrographs.

When the a plaster board is produced under pressure during setting from a certain direction (or dehydrated from a certain direction), crystals are formed like a plate and oriented producing a lamellar structure. This has been confirmed by electron-photomicrography and X-ray diffraction, as well. The electronphotomicrography, FIG. 3 of the section of a gypsum molded product which has been produced from B-form gypsum under application of pressure in accordance with this invention shows that the crystals are in a lamellar structure which comprises laminas arranged perpendicular to the direction of the pressure, which is clearly different from the conventional crystal structure obtained from B-form calcined gypsum which is needle-like or fine plate-like as shown in FIG. 2 and obtained from a-form calcined gypsum which is particlelike or pillar-like as shown in FIG. 1.

X-ray diffraction charts of the above sample with the incidence angles of the direction in parallel with the direction of the pressure, namely perpendicular to the surface of the molded product (hereinbelow called Sp direction), and the direction perpendicular to the direction of the pressure, namely in parallel with the surface of the molded product (here-in-below called S direction) are given as FIG. 8 and FIG. 9.

According to the S diffraction, chart, FIG. 8, the most intensive diffraction appears at the point of 20 ll.69 (d=7.56A), while according to the S diffracheating in the presence mixture of II--CaSO and III-CaSO tion, the diffraction at the point of 20 ll.69 is quite weak. Additionally, by comparing the diffraction intensities at other points, it is apparent that the (010) plane is selectively oriented in parallel with the surface of the molded product when pressure has been applied.

As shown above, the products produced according to this invention have Iamellar structure in which the crystal plane is oriented in a certain direction. Therefore, the present product of this invention has improved physical properties which have heretofore never been attained.

In the meantime hydraulically setting inorganic powder materials for example, portland cement, highsulphate slag cement, white cement, magnesia cement, and the like contain 3-l0 wt percent of calcined gypsum.

Since the mixtures of these hydraulically setting materials and water show excessive contraction when sand, gravel or some other material as an aggregate is not added, these have not been practically used. In addition to the above when gravel or sand is added into the hydraulically setting material and kneaded with water to plaster a wall with a hawk or such a mixture is cast in a mold, the free water remains within the plaster, so it takes a long time to set, cure, and dry. Thus, it is not applicable for mass production.

Up to present the ratio of the hydraulically setting inorganic material such as cement as listed above and water and sand has been selected experimentally. For example, cement mortar comprising cement, sand, and water, the quantity of water is 20-30 percent by weight of the cement in the case of thick mortar, and -70 percent by weight of the cement in the case of thin mortar. Such mortars are then cured in practice.

This invention is also applicable for using the above mentioned hydraulically setting inorganic powder material containing calcined gypsum. According to this invention, a strong product will be produced by the steps ofadding such quantity of water as more than the theoretical quantity necessary to set the hydraulically setting inorganic powder material and enough to keep the fluidity of the mixture, and setting the mixture under pressure, at the same time removing the excess water.

In this case, a product with practical strength is produced from the mixture of hydraulically setting inorganic powder material and water without an aggregate such as sand or gravel. Further, the process of curing and drying is not necessary.

Furthermore, in this invention, .it is possible to use gypsum in combination with one or more selected from any of hydraulically setting inorganic powder material such as white cement, portland cement, magnesia cement, furnace cement, mortar, etc.

Particularly, when gypsum and a cement such as white cement or portland cement as hydraulically setting inorganic powder material are employed in a weight ratio of gypsum of more than 30 percent to the total weight of gypsum and cement, the product will be stronger than that made solely of the hydraulically setting inorganic powder material.

The term hydraulically setting inorganic powder material herein used is thus intended to cover any such hydraulically setting inorganic material in a powder form as gypsum, plaster, hydraulic cements, and mixtures thereof; and preferably gypsum-based ones such as gypsum, plaster, and mixtures of gypsum and a hydraulic cement containing more than 3 percent, preferably 20 percent by weight of the mixture of the two. Portland cement usually contains some percentages, e.g., 3 percent by wt. of gypsum.

FIG. 10 and FIG. 11 show X-ray diffraction charts for the molded product as one embodiment of the present invention, which product has been produced from the mixture of gypsum and portland cement in 1:1 weight ratio. FIGS. 10 and 11 are the same in nature as FIGS. 8 and 9 except that the hydraulically-setting material consists solely of gypsum for FIGS. 8 and 9 while it consists of the mixture of gypsum and portland cement. Similar diffraction patterns can be seen even when gypsum is in admixture with portland cement in l:l weight ratio. Electron-photomicrographs of the molded product produced under pressure from a mixture of gypsum and portland cement in H weight ratio show some lamellar structures but the structures are not so distinct.

Additionally, in this invention an inorganic or organic filler may possibly be added into the hydraulically-setting inorganic powder material for the purpose of (1) improving the strength of the molded products (2) giving variation on the appearances, (3) adjusting the specific gravity.

The inorganic filler may be selected from known inorganic filler materials such as: bauxite, fluorite, cryolite, graphite, artificial graphite, colloidal graphite, silica, flint pebbles, siliceous sand, sulfur, barite, alumite, borax, fire clay, aluminous shale, sillimanites, agalmatolite, fire silica, dolomite, Magnesite, peridotite, feldspar, potteny stone, wollastonite, lithium mineral, asbestos, mica, rock crystal, quartz, calcite, garnet, corundum, gilsonite, vermiculite, perlite, pumice, pozzolana including fly ashes, natural slate, swollen shale, kaolin, talc, bentonite, acid terra alba, diatomaceous earth, zeolite, metal minerals, andesice, grnite, graywacke, serpentine, quartz gabbro, clay slate, volcanic ashes, metal powders, rock wool, glass fiber, carbon fiber, Metal whiskers, halloysite, montmorillonite, slag, kaoline fiber, red mud, sepiolite, attapulgite, palygorskite, glass pieces, concrete pieces.

The-organic filler may be selected from known organic filler materials such as: wool powder, wood chips hemp, wood wool, pieces of stretched plastic film, stretched fiber of plastics, especially those containing such an inorganic material, as a filler, pieces of foamed plastics, pieces of plastics, paper, rubber, bamboo, fabrics, rush, bagasse piece, bark of 'peanut, and plastic oligomers.

The quantity of the inorganic or organic filler added to the hydraulically-setting inorganic powder material is preferably below 50 percent by weight of the total weight of the mixture. When the content of gypsum or the hydraulically-setting inorganic powder material is below 3 percent by weight, the products cannot have sufficient strength, since such a small quantity of gypsum is not enough to bind all of the inorganic or organic filler within the product.

The gypsum product according to this invention is typically a plaster board, particularly a board product with 2 to 50 mm of thickness. It is, or course, possible to mold it into other shapes such as column. rectangular cylinder, block etc. These molded products may be either equal or unequal as to their inner organization or texture or as to their chemical composition. An example for the latter is a product comprising pure gypsum surface and core made of something other than pure gypsum such as white cement. As stated above, the product may possibly have a surface made of a porous material such as paper and fabrics adhered thereto.

Now, other exampled follow for a better understanding of this invention.

EXAMPLE 5 After mixing and stirring white cement and water in weight ratio of 2:1, and pouring the mixture into the same mold as described in Example 2, it was molded under pressure as shown in Table 6 for 5 minutes, and the molded product was taken out of the mold. Results of the tests on the product left as it was for a day are shown in Table 6.

Although it has long been impossible to get a sufficiently strong product made of cement and water, the present method makes it possible to produce a sufficiently satisfactory product for practical use. The mixture left for natural setting without pressure turned out to form no particular shape.

As understood from Examples 1,2,3 and 4, the specific gravity of the product may be adjusted by varying the pressure.

EXAMPLE 6 After mixing and kneading B-form calcined gypsum, white cement and water in a weight ratio of 1:122, and pouring the mixture into the same mold as described in Example 2, it was caused to set under pressure of kg/cm for 5 minutes, and product A with a specific gravity of 2.0 was produced.

A mixture of B-form calcined gypsum and water in a weight ratio of 1:1 were respectively molded in the same condition as above, and product B with a specific gravity of 1.9 and product C with a specific gravity of 2.1 were respectively produced.

Further, a kneaded mixture of B-form calcined gypsum, white cement, and water in a weight ratio of 1.122 was poured into the above mentioned mold and was left for natural setting, the setting time being 60 minutes, without pressure. Product D was thus produced.

These four products were tested as to their compression strength and bending strength in connection with the elapsed time, and the results are shown in FIG. 4 and FIG. 5.

In this regard, the starting point of the elapsed time is the time fifteen minutes after when the molded product was taken out of the mold.

molded products were produced. Molded products left for natural setting without pressure for 4 days also produced. Products 1 day after the setting were tested as to their bending strength and compression strength,

and the results are shown in Table 9. EXAMPLE 7 Table 9 A mixture of white cement and water in a weight f 1.2 k d d d f Run Pressure Quantlty, Bending Compression l' I was e an .3 orm calcmed gyp No. kg/cm Diatom.earth Sp.Gr. Strength Strength 1n the ratio shown 1n Table 7 was added into the mixkg/cmz kg/cm ture.-Then, it was poured into the same mold as in Ex- I 2 l 150 1.8 130 332 ample 2.and kept under pressure of 150 kg/cm and 2 5 85 1,6 52 1 125 the mold was caused to set for 5 minutes during which 3 150 90 L6 43 the excess waterwas removed. g 8 The product was taken out of the mold, and was 6 0 89 0.3 3 9 tested as to the bending strength after 10 minutes and Z 8 2 3 after 1 day. Results are shown in Table 7.

Table 7 Wl/Yn Calcined gypsum in the total Specific Bending Strength No. of white cement and calcined gypsum Gravity 10 min. 1 day EXAMPLE 8 EXAMPLE 10 A complex mono-filament made of poly propylene and portland cement dispersed therein was added at 2 wt percent into a kneaded mixture of B-form gypsum, portland cement, red mud, and water in a weight ratio of l:l:3:2, and the mixture was poured into the same mold as in Example 2. Then molded products with various specific gravity in accordance with the variation of the pressure were produced under pressure through a pressuring period of 5 minutes (but water discharge period of 10 seconds) of setting.

The products were tested as to their bending strength and compression strength one day after the molding under pressure, and the results are shown in Table 8.

In this connection, products left for natural setting without pressure had so many cracks that it was impossible to get a specimen for the test.

Seven percent by weight of a blend of an asbesto and wood powder in a weight ratio of 5:1 was added into a kneaded mixture of B-form calcined gypsum, white cement, and water in a weight ratio of 1:1 :13, and then sintered diatomaceous earth was added at 5, 80, 90 and 91 wt percent, and the mixture was poured into the same mold as in Example 2. Then, it was caused to set under pressure of kg/em for 5 minutes, (water bein dischar ed within 10 seconds) and various An apparatus shown in, FIG. 6 was used, a liquid slurry of calcined gypsum and water in a weight ratio of 2:1 (1:25 in mole ratio) was gypsum and water in a weight ratio of 2:l (2.5 times the theoretical quantity of water) was made of the sintered porous metal 1 as shown in FIG. 6, and the capacity of the room 3 of the female mold was reduced under pressure of 50 kg/cm for 10 seconds by the male mold 5 provided with a relief design having lmm of depth at the largest and the largest taper 0=89 in the inside 4 whereby the excess water was removed through the sintered metal wall. Then, the solidified product was taken out.

The product had a convex relief design precisely corresponding to that of the mold, a specific gravity of 2.0, compression strength of 461 kg/cm and bending strength of 59 kg/cm EXAMPLE 11 An apparatus shown in FIG. 6 was used a liquid slurry of calcined gypsum and water in a weight ratio of 2:1 was poured into the room 3 of the female mold where the bottom is made of sintered porous metal 1 as shown in FIG. 6, and the capacity of the room 3 of the female mold 2 was reduced under the pressure of 50 kg/cm for one minute (water discharged within 10 seconds) by the male mold 5 provided with a relief design having 1mm ofdepth at the largest and the largest taper 0=92 in the inside 4 being intervened by polyethylene film having a thickness of 20011.. After pressing and dehydration, the male mold and the film was taken out. Then the solidified product was obtained. The product had a convex relief design precisely corresponding to that of the male mold and a specific gravity of 1.9, compression strength of 447 kg/cm and bending strength of 56 kg/cm 1f the polyethylene film is not employed, the relief desion Pnrrecnnnrlino tn thp hart nf tannr 1min" mnro floor 13 90 is partly broken and the product does not have a good appearance.

As illustrated in Example 10, when the taper is beyond 90 (or so-called reverse taper), the molded product will be partly broken and show a bad appearance when it is demolded from a mold unless flexible film is placed between the product and the mold.

Accordingly, when a relief design by the reverse taper is required, or otherwise the taper stands between 87 and 90 or the relief design has considerable depth,

chipped products will be formed unless use is made of flexible film sheet is placed between the product and the mold.

Such film may be a flexible film with a thickness of, for example, 0.02 to, 1mm preferably 0.1 to 0.3 mm, of such as low density polyethylenes, ethylene-vinyl acetate copolymers, plasticized polyvinyl chloride, vinylidene chloride. cellophane, vinyl chloride-vinylidene chloride copolymers, high density polyethylenes, ny- Ion, polyester resin. and ethylene-propylene copolymers.

A relief design by reverse taper is well formed, if the width L, of the entrance of the cavity of the mold 5 and the length L: of the cavity at the broadest part are in accordance with the following relation as shown in H6. 7.

What we claim is:

l. A molded gypsum product which comprises gypsum crystals having a lamellar structure.

2. A molded gypsum product according to claim 1 wherein said product has a core made of B-form gypsum.

3. A molded gypsum product according to claim I wherein said product is in the form of a board.

4. A molded gypsum product the gypsum component of which has a crystalline structure by X-ray diffraction, said crystalline structure being characterized in that (010) planes thereof are arranged in one direction.

5. A molded gypsum product according to claim 4 wherein said (010) plane direction is perpendicular to the direction of application of pressure which has been applied to the molded gypsum product during the molding.

6. A molded gypsum product according to claim 4 wherein said product is made substantially solely of gypsum.

7. A molded gypsum product according to claim 4 wherein said product is made of an intimate mixture of gypsum and a hydraulically setting inorganic powder material.

8. A molded gypsum product according to claim 7 wherein said hydraulically setting inorganic powder material is a member selected from the group consisting of white cement, portland cement, magnesia cement, blast-furnace cement, and plaster in a weight ratio of said gypsum to the total of said gypsum and the hydraulically setting inorganic powder material of not less than 30 percent by weight. 

2. A MOLDED GYPSUM PRODUCT ACCORDING TO CLAIM 1 WHEREIN SAID PRODUCT HAS A CORE MADE OF B-FORM GYPSUM,
 3. A molded gypsum product according to claim 1 wherein said product is in the form of a board.
 4. A MOLDED GYPSUM PRODUCT THE GYPSUM COMPONENT OF WHICH HAS A CRYSTALLINE STRUCTURE BY X-RAY DIFFRACTION, SAID
 5. A molded gypsum product according to claim 4 wherein said (010) plane direction is perpendicular to the direction of application of pressure which has been applied to the molded gypsum product during the molding.
 6. A molded gypsum product according to claim 4 wherein said product is made substantially solely of gypsum.
 7. A molded gypsum product according to claim 4 wherein said product is made of an intimate mixture of gypsum and a hydraulically setting inorganic powder material.
 8. A molded gypsum product according to claim 7 wherein said hydraulically setting inorganic powder material is a member selected from the group consisting of white cement, portland cement, magnesia cement, blast-furnace cement, and plaster in a weight ratio of said gypsum to the total of said gypsum and the hydraulically setting inorganic powder material of not less than 30 percent by weight. 